Method and apparatus for performing frame synchronization in an asymmetrical digital subscriber line (ADSL) system

Abstract

A method and apparatus for performing DMT frame synchronization in an ADSL system begins by providing a training signal (82) to a receiver of the ADSL system (34). The training signal (82) is processed to result in a desired impulse response. The impulse response is used to reduce inter-symbol interference between time-adjacent DMT frames (100-108). The desired impulse response is used to calculate a frame misalignment value (ΔT). The frame misalignment value (ΔT) of the desired impulse response (84) is then utilized to adjust an internal counter of the receiver to perform frame alignment. The use of the training signal (82) and impulse response (84) to provide for both intersymbol interference reduction (FIG. 5) and frame synchronization (FIG. 8) provides for fast ADSL initialization.

What is claimed is: 
     
       1. A method for performing frame synchronization and inter-symbol interference reduction in a communication system, the method comprising the steps of: receiving a training signal at a receiver of the communication system, the receiver having a counter which counts from one to N, where N is a finite integer defining a block of data;   digitally sampling the training signal to form a sampled training signal;   providing the sampled training signal to the counter;   the counter providing blocks of the sampled training signal to a time domain equalizer (TEQ);   training the TEQ, whereby a desired impulse response is determined which ensures that any inter-symbol is within a specified tolerance;   determining a first value of the counter corresponding to a starting edge of the desired impulse response during a first block of the sampled training signal; and   determining a frame misalignment value (Δ) using the desired impulse response and the first value of the counter; and   adjusting the counter using the frame misalignment value (Δ) to achieve frame synchronization within the communication system.   
     
     
       2. The method of claim 1 wherein the communication system is an asymmetric digital subscriber line (ADSL) system. 
     
     
       3. The method of claim 1 wherein the counter is reset to a starting value when the frame misalignment value (Δ) is detected within the counter whereby frame alignment is achieved. 
     
     
       4. The method of claim 1 wherein the counter repeatedly counts from an initial value to N wherein N is a finite integer, a unique value of the counter identifying a unique symbol sample time period for one symbol of data communicated in the communication system. 
     
     
       5. The method of claim 1 wherein the step of digitally sampling the training signal is used to configure the time domain equalizer so that inter-symbol interference is minimized within the communication system. 
     
     
       6. A method for performing frame synchronization and reduction of inter-symbol interference in an asymmetric digital subscriber line (ADSL) system, the method comprising the steps of: providing a training signal to a receiver of the asymmetric digital subscriber line (ADSL) system, the receiver having a counter which counts from one to N to enable sampling of the training signal at selected intervals within the receiver;   digitally sampling the training signal via an analog to digital converter located within the receiver, digital sampling being performed at the selected intervals defined by the counter and resulting in digitally sampled data;   deriving a desired impulse response from the digitally sampled data using frequency domain analysis;   iteratively changing equalizer settings of an equalizer internal to the receiver to iteratively reduce any inter-symbol interference within the training signal to within a specified tolerance;   converting the desired impulse response from frequency domain to time domain to result in a time domain desired impulse response;   determining a frame misalignment value (Δ) using the time domain desired impulse response and the counter, wherein the frame misalignment value (Δ) corresponds to a count of the counter on a starting edge of the desired impulse response during a first sample of the training signal; and   adjusting the counter using the frame misalignment value (Δ) and the count of the counter to achieve frame synchronization within the asymmetric digital subscriber line (ADSL) system.   
     
     
       7. The method of claim 6 wherein the training signal comprises a plurality of frequency carriers at different frequencies, each frequency carrier in the plurality of frequency carriers communicating at least one bit of data. 
     
     
       8. The method of claim 6 further comprising the additional steps of: terminating the training signal; and   receiving a data signal instead of the training signal wherein the step of changing a value in the counter ensures proper alignment and sampling of the data signal within the asymmetric digital subscriber line (ADSL) system.   
     
     
       9. The method of claim 8 wherein N is changed to M wherein M is a finite integer greater than N whereby symbols associated with the data signal are greater in time length than symbols associated with the training signal. 
     
     
       10. The method of claim 6 wherein the step of changing a value in the counter comprises: resetting the counter to a starting value when the frame misalignment value (Δ) is detected as a value stored in the counter.   
     
     
       11. The method of claim 6 wherein the step of determining a frame misalignment value (Δ) comprises: identifying a point on the time domain desired impulse response; and   storing a counter value resident within the counter when the point is detected on the time domain desired impulse response, the counter value being used as the frame misalignment value (Δ).   
     
     
       12. A method for synchronizing data frames in an asymmetric digital subscriber line (ADSL) system having a time-domain equalizer (TEQ) that is iteratively trained using a desired impulse response filter and having a sample counter, the method comprising: converting a frequency-domain impulse response generated by the desired impulse response filter to a time-domain impulse response;   correlating the time-domain impulse response to a sample counter sequence;   determining a sample counter value associated with a starting edge of the time-domain impulse response;   reading a first sample counter value;   comparing the first sample counter value to the sample counter value associated with the starting edge of the time-domain impulse response; and   if the first sample counter value is equal to the sample counter value associated with the starting edge of the time-domain impulse response resetting the sample counter to the sample counter value associated with the starting edge of the time-domain impulse response.   
     
     
       13. A method as in claim 12, wherein the step of converting the frequency-domain impulse response is done using an Inverse Fast Fourier Transform. 
     
     
       14. A method as in claim 12, further comprising the step of: if the first sample counter value is not equal to the sample counter value associated with the starting edge of the time-domain impulse response: reading a second sample counter value from the sample counter;   comparing the second sample counter value to the sample counter value associated with the starting edge of the time-domain impulse response; and   if the second sample counter value is equal to the sample counter value associated with the starting edge of the time-domain impulse response resetting the sample counter to the sample counter value associated with the starting edge of the time-domain impulse response.     
     
     
       15. A method as in claim 12, wherein the time-domain equalizer (TEQ) iterative training includes the following steps: the desired impulse response filter receiving a training sequence, x(t);   the time-domain equalizer (TEQ) receiving a distorted training sequence, x'(t) which is a distortion of the training sequence, x(t);   the TEQ providing a first equalized sequence;   the desired impulse response filter providing a filtered training sequence;   determining a first difference value by comparing the first equalized sequence to the filtered training sequence; and   if the first difference value is not within a predetermined convergence value adjusting internal parameters of the TEQ.   
     
     
       16. A method as in claim 15, further including the step of: if the first difference value is not within a predetermined convergence value adjusting filter parameters of the desired impulse response filter.   
     
     
       17. A frame synchronizer for synchronizing data frames in an asymmetric digital subscriber line (ADSL) system, the frame synchronizer having a counter which allows data frames to be sampled a plurality of times, the frame synchronizer comprising: a time-domain equalizer (TEQ) which receives a channel distorted signal;   means for processing the channel distorted signal to change equalizer values within the time-domain equalizer (TEQ) of the asymmetric digital subscriber line (ADSL) system to reduce inter-symbol interference of the channel distorted signal, the means for processing providing a frequency domain response;   means for converting the frequency domain response to a time domain response in order to determine a frame misalignment value Δ from the time domain response, the frame misalignment value Δ being a difference between a starting value of the counter and selected time point of the time domain response; and   adjusting circuitry for changing a starting point of a counter sequence of the counter so that sampling of data frames is synchronized.   
     
     
       18. A frame synchronizer as in claim 17, wherein the frequency domain response is a desired impulse response. 
     
     
       19. An asymmetric digital subscriber line (ADSL) frame synchronizer stored on computer readable medium, the frame synchronizer comprising: computer instructions used to digitally sample a training signal provided as input to the frame synchronizer, the sampling being performed at selected intervals to derive a desired frequency domain impulse response, the desired frequency domain impulse response ensuring that any inter-symbol interference within frames of the training signal is within a specified tolerance;   computer instruction used to convert the desired frequency domain impulse response to a desired time domain impulse response   computer instructions used to determine a frame misalignment value (Δ) from the desired time domain impulse response; and   computer instructions used to adjust the counter to compensate for the frame misalignment value (Δ) to achieve frame synchronization within an ADSL system.